Because cutting inserts attached to an indexable rotary cutting tool are usually made of expensive WC-Co-based cemented carbide, cutting inserts having point-symmetrical polygonal plate shapes have been conventionally used to use cutting edges formed along edge lines where upper surfaces (rake faces) are crossing side surfaces (flanks), as efficiently as possible. In the case of a square shape, for example, four cutting edges can be used by turning four times.
To increase cutting edge regions, so-called negative (double-sided) cutting inserts have been proposed. Because a negative cutting insert has cutting edges on upper and lower surfaces, after all cutting edges on the upper surface are worn, the cutting insert is turned upside down and reattached to a tool body to use cutting edges on the lower surface for cutting. Accordingly, the negative cutting insert has as many cutting edges as 2 times. As such negative cutting inserts, various ones have been proposed.
JP 2001-157904 A proposes a cutting insert having an octagonal shape comprising an auxiliary cutting edge and a main cutting edge in each corner region, its upper and lower surfaces having eight main cutting edges each, and 16 main cutting edges in total. JP 2006-224278 A proposes a polygonal-plate-shaped, cutting insert comprising a main cutting edge in each side, and a pair of auxiliary cutting edges via a flank in each corner, which can be used 16 times when turned clockwise and counterclockwise. JP 2010-142948 A proposes a square-plate-shaped, negative cutting insert comprising a main cutting edge in each side, and an auxiliary cutting edge in each corner, with a flank having an acute angle to a centerline between a pair of auxiliary cutting edges. JP 2010-536599 A proposes a square-plate-shaped, negative cutting insert capable of conducting precision cutting, which comprises auxiliary cutting edges each extending along a projecting curved edge line having a radius R of curvature between an upper or lower surface and a side surface, and main cutting edges each extending along a edge line between an upper or lower surface and a corner surface, both ends of each main cutting edge being connected to the auxiliary cutting edges. JP 2011-51029 A proposes a polygonal-plate-shaped, negative cutting insert having main cutting edge portions in side portions, and auxiliary cutting edge portions in corner portions. However, these cutting inserts do not have both rough-cutting edge portions and finish-cutting edge portions.
JP 2000-503912 A discloses, as shown in FIG. 19, a hexagonal-plate-shaped cutting insert 110 comprising six sides and six corners 115, the sides comprising rough-cutting edge portions 113 and fine-cutting edge portions 114 alternately, such that it can conduct both rough cutting and fine cutting. However, because this cutting insert 110 has both rough-cutting edge portions 113 and fine-cutting edge portions 114 in the sides, it cannot use the portions 115 for fine cutting. Accordingly, the cutting insert 110 of JP 2000-503912 A cannot be used for the cutting of molding die cavities, etc.
U.S. Pat. No. 7,410,332 discloses, as shown in FIG. 20, a substantially square-plate-shaped, face-milling insert 200 comprising finish-cutting edge portions 201 and rough-cutting edge portions 202. Both finish-cutting edge portions 201 and rough-cutting edge portions 202 are formed in side portions, a finish-cutting edge portion 201 in one side comprising a pair of cutting edge portions 201a, 201b (201c, 201d)connected via a corner 207a with an angle β, and a rough cutting edge portion 202 in one side comprising a pair of cutting edge portions 202a, 202b (202c, 202d) connected via a corner 207bwith angle β′. Accordingly, this face-milling insert 200 is not 90° symmetrical but 180° symmetrical with respect to a mounting center hole 209. Accordingly, the number of finish-cutting edge portions and rough-cutting edge portions in one cutting insert is insufficient.